You can just add meta field on meta class for the interest value. Do you have to implement any extra logic on that? Can you give an high level idea abot your requirements?

if I understand you correctly you want to override price calculation of a variant. For variant "Interest" you want to do a custom calculation to get pricing.

you need to write your own pricing service (or price helper class) that will inherit with DefaultPlacedPriceProcessor & IPlacedPriceProcessor

example snippet

public class PlacedPriceProcessor : DefaultPlacedPriceProcessor, IPlacedPriceProcessor
    {
        public override bool UpdatePlacedPrice(ILineItem lineItem, CustomerContact customerContact, MarketId marketId, Currency currency, Action<ILineItem, ValidationIssue> onValidationError)
        {
            // Custom logic
        }

        public override Money? GetPlacedPrice(EntryContentBase entry, decimal quantity, CustomerContact customerContact,
            MarketId marketId, Currency currency)
        {
             // Custom logic
        }

  }